Generator Wattage Calculator (running & starting watts)

Understanding the generator wattage calculator — concept and principle

This calculator adds up the running and starting watts of the appliances you want to power at the same time and tells you the minimum running and surge ratings a generator needs. Use it before buying a portable or standby generator for power cuts, camping or work sites.

Running plus the biggest surge: with the default inputs the result is 5,170 W starting (choose a generator above this).

How it is calculated

Each line is an appliance: name, running watts, and optional extra starting watts.

Running watts needed = sum of the running watts of everything on the list.
Starting (surge) watts needed = running watts needed + the largest single extra starting wattage.
The calculator names the appliance with the biggest surge.

The principle

Motors in fridges, pumps, fans and air-conditioners draw a short surge of power when they start, for a second or two, on top of their running power. Resistive loads such as heaters, kettles and lights draw no extra at start. Motors rarely start at exactly the same instant, so the usual method adds only the single largest surge to the total running load. A generator has a continuous (running) rating and a short-term surge rating; both must exceed your figures, or the voltage will sag, the breaker will trip, or a motor may stall.

Worked example

Default list: refrigerator 700 W (+2,200), sump pump ½ HP 1,050 W (+2,200), furnace fan ⅓ HP 700 W (+1,400), LED lights 60 W, TV 400 W, laptop 60 W.

Running watts = 700 + 1,050 + 700 + 60 + 400 + 60 = 2,970 W (2.97 kW).
Largest extra starting watts = 2,200 (the fridge and the pump tie; the first listed, the Refrigerator, is named).
Starting watts = 2,970 + 2,200 = 5,170 W.
Appliances counted = 6.

How to read the result

Choose a generator whose running rating is above 2,970 W and whose surge rating is above 5,170 W, with some headroom; running a generator constantly near its limit shortens its life. You can lower the requirement by staggering loads, such as not starting the pump while the fridge compressor starts. Check the actual wattage on each appliance's rating plate, because typical values vary a lot.

Common mistakes

Adding every appliance's starting watts together, which oversizes the generator.
Forgetting the surge of motors altogether.
Using kVA as if it were kW.
Connecting a generator to house wiring without a proper transfer switch.

Keep in mind: For estimates only. Generators produce deadly carbon monoxide: never run one indoors, in a garage or near windows. Connections to house wiring must follow local rules and be made by a qualified electrician.

Frequently asked questions

What size generator do I need to run a fridge?

A typical fridge runs at about 700 W but needs about 2,200 W extra to start, so allow roughly 2,900 W of surge for it alone.

What is the difference between running and starting watts?

Running watts keep an appliance going; starting watts are the brief extra power motors need to start.

Should I add all the starting watts?

Usually only the largest one, because motors seldom start at the same moment.

Sources: Eugene Water & Electric Board (EWEB), "Generator sizing considerations" · Cummins, "What size portable generator do you need?" (2025)

Checked 9 Nov 2026. Eugene Water & Electric Board (EWEB), “Generator sizing considerations” (method: add running watts, then add the highest additional starting watts; typical wattage table); Cummins, “What size portable generator do you need?” (2025).

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